I think it's one of these 3 things...

1. Nute burn (don't think so, most of the leaf tips look fine)
2. pH is off (probably not, always feed at 5.8 and meter is calibrated and checked against drop test kit as well)
3. Dirty tap water (ppms run 140-150 out of the faucet, locals are encouraged not to drink it directly)

After the yellowing & brown spot appeared a two days ago, I started using filtered water (from a machine). The ppms after filtering went down to 7.5. I have no idea what's in the water as I'm not in my home country.

Beyond those three possibilities, I'm completely stumped. It's C99 from Female Seeds and I know it's quite a light feeder, although I feel I haven't been giving it too much. It's been started from seed in Botanicare coco (from a brick) and I've mixed it with 30% rice hulls. Two and a half weeks since popping, weird tri-leaf formation too. I'm going to wait and see if the filtered water doesn't make a positive change. In the meantime...

Any thoughts? Much appreciated.

This was three days ago, shortly after I first noticed the problem:



...and this is today:



It seems like the damage has been contained and growth is continuing, right? Should I trim the yellowed leaves and hold tight?

I've always been told never trim unless more than 50% of the leaf is dead.

You have yourself a trifolate plant my friend! C99 is known to do shit like this. I have grown out a trifolate afgooey years ago and although cannot take heavy feedings well whatsoever was the most potent plant I've ever grown. Yield was low as well. It basically a mutant brother. I promise mutants can be worth growing out

Yeah, gman, I heard similar things about tri-plants. Good to hear your experience. I'll hold off on any trimming now at such an early stage and keep my fingers crossed. May the ganja gods bless me with power of potency!